Planktonic Foraminifera As Oceanographic Proxies: Comparison of Biogeographic Classifications Using Some Southwest Pacific Core-Top Faunas
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Hindawi Publishing Corporation ISRN Oceanography Volume 2013, Article ID 508184, 15 pages http://dx.doi.org/10.5402/2013/508184 Review Article Planktonic Foraminifera as Oceanographic Proxies: Comparison of Biogeographic Classifications Using Some Southwest Pacific Core-Top Faunas G. H. Scott GNS Science, 1 Fairway Drive, Lower Hutt 5010, New Zealand Correspondence should be addressed to G. H. Scott; [email protected] Received 29 April 2013; Accepted 17 June 2013 Academic Editors: M. Elskens and M. T. Maldonado Copyright © 2013 G. H. Scott. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The distribution of planktonic foraminifera, as free-floating protists, is largely controlled by hydrography. Their death assemblages in surficial sediments provide proxy data on upper water mass properties for paleoceanography. Techniques for mapping faunal distributions for this purpose are compared in a study of 35 core-top samples that span the Subtropical Front in the Southwest Pacific. Faunas are analyzed by taxon composition, order of dominant taxa, and abundance. Taxon composition (presence-absence data) and dominant taxa (ordinal data) recognize groups of sites that approximate major water mass distributions (cool subtropical water, subantarctic water) and clearly define the location of the Subtropical Front. Quantitative data (relative abundances) more closely reflect the success of taxa in upper water mass niches. This information resolves groups of sites that reflect differences in intrawater mass hydrography. Comparisons suggest that abundance data should provide much better oceanographic resolution globally than the widely used ordinal biogeographic classification that identifies only Tropical, Subtropical Transitional, Subpolar and Polar provinces. As the data are strongly structured by variance in the abundance of Globigerina bulloides, Globorotalia inflata, Neogloboquadrina incompta,andNeogloboquadrina pachyderma, comparable classifications result from most clustering strategies. Principal coordinates analysis best represents the configuration of sites in two dimensions. 1. Introduction planktonic foraminifera, Imbrie and Kipp [3]usedQ-mode factor analysis (QFA; [6]) to reduce the dimensionality of Although the diversity of the Holocene planktonic foraminif- n-taxoncore-topfaunalcountstoasmallnumberoffactor eral fauna is modest, many taxa are distributed through the assemblages. Each is a statistically distinct linear combination world ocean [1] and are exemplars for the interpretation of of the n-taxa. This simplifies the regression equation and Cenozoic oceans [2]. For paleoceanography, a primary goal is aids its interpretation. The ecological relevance of each fac- to classify Holocene faunas on the basis of their distributions tor assemblage was assessed by plotting the geographical andtorelatethemtothehydrography.Thebiogeographyof distribution of its contribution to each core-top fauna to show a single taxon can be simply mapped. The organization of sites according to their faunal content in a manner that is its relation to hydrography. While this provides a perspective informative for paleoceanography is a more complex task that on faunal biogeography, QFA does not identify discrete can be approached at several levels of analysis. Seminal works geographic/areal units based on faunal similarity. Rather, the in this field are those by Imbrie and Kipp3 [ ]andBe[´ 4]. underlying concept is a mixture model wherein a small num- Although they are widely applied and are similarly focused ber of notional source assemblages contribute to each core- on the distribution of dominant taxa, little attention has been top fauna. given to their quite different methods and to their suitability Much simpler methods were used by Be´ and Tolder- for resolving faunal biogeographies [5]. lund [7]andBe[´ 4]. From large databases of census data In developing a regression-based methodology for esti- from surface tows and sediment samples, they mapped mating sea surface temperatures (SST) from abundances of distributions of the major species and found regions where 2 ISRN Oceanography they occurred most abundantly. These data were integrated Local oceanic circulation tends to dominate the oceanic into a classification of five biogeographic provinces in the environment east of New Zealand. Subtropical water flows world ocean, principally defined by dominant taxa. The around the tip of North Cape and forms the south-eastward provinces correspond broadly to major global hydrographic flowing East Auckland Current [16]. This current extends as regions [8] and are bi-hemispheric. Although methodology far as East Cape before forming the southward-flowing East was not detailed, it transforms counts (quantitative data) to Cape Current [11, 17],whichisdeflectedeastwardbyChatham ranks (ordinal data). This simplifies the analysis of complex Rise. In this northern region, upper ocean circulation is abundance data. Ranks appear to have been determined also strongly influenced by three variable, semipermanent, by careful inspection of data rather than by an algorithm. anticyclonic eddies. In the south the Southland Current, As with QFA, dominant taxa are of primary importance, about 90% ASW [15]flowsaroundsouthernNewZealand but, in a conventional approach to biogeography, taxon and swings northward along the east coast of the South distributions are used to define discrete areal units. Unlike Island. Part of this flow turns eastward along the southern QFA, taphocoenses are interpreted as unitary faunas. flank of Chatham Rise, while a component continues north An important issue for planktonic foraminiferal biogeog- through Mernoo Saddle. Campbell Plateau is characterized raphy raised by the QFA method is whether it is realistically by weak mean flows but cyclonic flow is well developed portrayedwithamixturemodel.Aquestionraisedbythe around western Bounty Trough [18]. ordinal approach is whether there is significant loss of infor- mation when taxon abundances are reduced to ranks. These, and related topics, are addressed via the analysis of core-top 1.2. Previous Work. Kustanovich [19] recognized five bio- ∘ faunal data in the work of Weaver et al. [9] from a sector of geographic regions (Figure 2(a)) between 18–54 S, based on the Southwest Pacific east and south of New Zealand (36– species composition and dominance. Apart from the STF, no ∘ 61 S). The hydrography includes Subtropical Water (STW), relation between the hydrography and regional boundaries the Subtropical Front (STF), Australasian Subantarctic Water was noted. Eade [20] found that distributions of faunas in ∘ (ASW), and the northern front of the Antarctic Circumpolar plankton tows between 18–36 Swererelatedtotheprincipal Current (ACC). It provides a good basis for comparing faunal water masses. Several boundaries correspond closely to those regions with oceanography. of Kustanovich [19].Clusteranalysisoffaunalabundances in 234 surface sediment samples from the South Pacific by Parker and Berger [21]placedallfromtheNewZealand 1.1. Oceanographic Outline. Core-top samples come from a region in one group which spanned the STF (Figure 2(b)). region of diverse bottom topography and complex oceanog- This result may be related to the method used to calculate raphy spanning the transition from Subtropical to Sub- faunal similarity. antarctic water masses (Figure 1). There are two principal submerged continental blocks. Chatham Rise trends east for Although the scale of Be[´ 4,Figure7]preventsclose c.1300 km off central South Island. Depths at its crest are assessment, sites #1-2 of Weaver et al. [9] are in his Subtropical often <500m.SoutheastofSouthIslandisthevasttriangular Province. Remaining sites north of the STF and those in Campbell Plateau, mostly submerged between 500–1000 m. Bounty Trough are in the Transitional Province. The Polar Between these blocks is Bounty Trough, a failed rift. Bollons Province includes sites on Campbell Plateau. More details are Seamount rises over 2000 m above the abyssal Southwest shown in Figure 1 which treats line weights for taxa in Be[´ 4, Pacific basin off Campbell Plateau. Chatham Rise deepens Figure 8] as rank indices and applies them to Weaver et al. northward into Hikurangi Plateau, bordered on the west by [9, Table 3]. On this basis, seven of the eight faunas north theNewZealandplateboundary.Forcomparisonwithfaunal of Chatham Rise (Figure 1(a)) are identified as Transitional data,coresitesaregroupedintoNorthChatham,Bounty because Globorotalia inflata is the highest ranked species; Trough, and Campbell Plateau bathymetric regions. Site #35 Neogloboquadrina pachyderma and Globigerina bulloides are ∘ (61 S) is from the Southern Ocean. in second rank at several sites. Ten faunas in Bounty Trough The Subtropical Front (STF) lies over Chatham Rise. It is areallocatedtotheSubantarcticProvincebecauseGlobige- ∘ defined by surface temperature gradients of 4 C/200 km and rina bulloides is dominant. Neogloboquadrina pachyderma is strong along-rise currents [11–13]. Primary production in the in second rank in nine faunas. Six Campbell Plateau faunas frontal zone is high [14]. Flow from the north of warm, highly in which Globigerina bulloides is in first rank are classed as saline, nutrient-depleted Subtropical Surface Water (STW) Subantarctic. The remaining eight, with Neogloboquadrina ∘ extends south into